Phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidylglycerol (PG) and diphosphatidylglycerol (DPG) are among the major phospholipids found in plant tissues. The distribution of these lipids among the various organelles of different tissues and among different plants has been comprehensively studied. The pathways by which these lipids are synthesized have also been studied extensively but very few of the plant enzymes involved in these pathways have been purified or their corresponding genes cloned.
Choline phosphate cytidylyltransferase (also called CTP:choline phosphate cytidylyltransferase; E.C. 2.7.7.15) catalyzes the conversion of ethanolamine and choline phosphate to their respective CDP-aminoalcohols. Choline phosphate cytidylyltransferase is thought to regulate the flux through the CDP-choline pathway for PC biosynthesis. In animal and plant cell extracts the choline phosphate cytidylyltransferase enzymatic activity is found in the soluble and in the membrane fractions. It has been proposed that the animal and plant choline phosphate cytidylyltransferases are regulated by the lipid-promoted translocation of the enzyme from the cytosol to the endoplasmic reticulum (ER). In this scenario, the enzyme is inactive while in the cytosole and reversible phosphorylation results in binding to the ER membrane and activation of the enzyme.
cDNAs encoding the rat and yeast choline phosphate cytidylyltransferase proteins have been identified (Kalmar et al. (1990) Proc. Natl. Acad. Sci. USA 87:6029-6033; Tsukagoshi et al. (1987) Eur. J. Biochem. 169:477-486). Pea, rape, and castor bean cDNAs encoding choline phosphate cytidylyltransferases have also been identified (Jones et al. (1998) Plant Mol. Biol. 37:179-185; Nishida et al. (1996) Plant Mol. Biol. 31:205-211; Wang and Moore (1991) Plant Physiol. 96(suppl.): 126). Comparison of the amino acid sequences of the rat and yeast choline phosphate cytidylyltransferase show a highly conserved central region surrounded by divergent amino- and carboxy-terminal domains.